A variety of bulk products, primarily cement but others too, must be subjected to high temperatures during a stage of their manufacturing process. Cement ordinarily is produced by burning calcareous and argillaceous and other raw materials in a cement kiln to produce an interim stage called clinker. The clinker is later pulverized to form cement powder. The drying kiln ordinarily comprises a large rotating cylinder which is between 200 and 500 ft. long, and which is inclined slightly from horizontal. Raw materials are injected into one end of the cylinder, flow slowly downwardly through it and are agitated as they flow by the rotation of the cylinder. A burner projects a flame down the center of the cylinder to process the raw materials into clinker. From the discharge end the hot product drops gravitationally into a high temperature cooler for further processing.
The necessary heat is generated by one or more burners positioned within the discharge end of the rotary kiln. In the past these burners were usually gas or oil fired burners because of their ease of operation. With the ever increasing costs of such fuels and their increasing scarcity, they have become unattractive and such gas and oil burners are being converted into solid fuel, e.g. coal burners at an increasing rate because solid fuels are available at substantially lower costs.
The burners must be arranged so that the flame extends over a substantial distance, say from a minimum of 10 or 15 ft. to as much as 50-80 ft. or more from the discharge end into the kiln to heat the raw materials sufficiently to convert them into the desired product. The fuel itself is combusted in the kiln above the product carried therein. For gaseous and liquid fuels this presents no problem. For solid fuels, e.g. coal, it is necessary to first pulverize the coal so that it can be discharged from the burner into the kiln in the form of fine particles for combustion therein.
To accomplish this it has heretofore been common practice to pulverize the coal in a mill and entrain the coal in an airflow to convey the pulverized coal directly to the burner. Coal pulverizing mills require a significant amount of air and it was common to use the same air both for conveying the coal to the burner and as a source of primary combustion air.
Such a direct firing of the coal has several disadvantages. First, the coal mills typically require up to 45% of the combustion air depending on the coal. This is a relatively constant amount of air irrespective of the rate at which coal is pulverized. Thus, the coal to air ratio coming out of the mill is difficult to control when the burner load is changed and this complicates the necessary controls or contributes to combustion inefficiencies. Further, the air is moisture-laden, which increases with the moisture content of the coal. This adversely affects the combustion process and the maximum temperature that can be attained in the kiln. Accordingly, since this air is relatively cold such burners, when used for a process such as cement manufacturing have a low efficiency and are difficult to regulate.
To overcome these shortcomings an indirect firing system has heretofore been devised. In this approach, the coal is again pulverized in the mill but is then separated from the air required by the mill in a cyclone separator or the like. The air, after appropriate filtration is discharged while the coal is stored in a bin or storage container from which it can be withdrawn irrespective of the rate at which the coal is pulverized.
The pulverized coal from the bin is entrained in a coal conveying airstream at the desired rate. The stream transports the coal to the burner and normally constitutes the burner's source of primary combustion air.
This arrangement has several advantages over the direct firing system. For one, the coal and air feed rates are independent of the coal pulverizing mill. Secondly, the air used in the pulverizing mill and the moisture transferred to it from the coal as it is being pulverized are discharged so as to not adversely affect the combustion of the coal in the kiln and reduce the flame temperature. Nevertheless, this simple indirect firing system has several disadvantages, the most serious one being the difficulty of igniting and maintaining a flame because of the relatively low temperature of the coal being discharged by the burner and the relatively high volume of air employed to convey the coal to the burner, the latter constituting up to about 20% of the theoretical amount of air needed to combust the coal in the kiln. Other disadvantages experienced with this system are the large conduits that are necessary for conveying the relatively large air volume in which the coal is entrained to the burner, the resulting large size of the burner, etc. which made the overall installation not only expensive but more difficult to maintain.
Attempts have also been made to add to the pulverized coal-airstream additional and heated, supplemental primary air so as to raise the temperature of the stream in the burner to thereby facilitate the combustion of the coal in the kiln and raise the flame temperature. Although such attempts were marginally helpful, ignition difficulties persisted primarily because it was not practicable to add a sufficient amount of heated supplemental air to raise the temperature of the coal to a point where volatiles are driven off, i.e. vaporized so that they can later be flash-ignited, which would help bring the temperature of the non-volatiles in the coal to their flash point.
A by-product of the ignition difficulties experienced in the past is that it was heretofore not feasible to use solid fuels having no or only a low content of volatiles, such as petroleum coke, which comprises almost exclusively carbon, because the necessary ignition temperature could not be reached with pulverized coal burners heretofore available. Yet, such low volatile solid fuels constitute a readily available, low cost source of energy and they would otherwise be ideally suited for use in kilns of the type here under consideration.